Actuator controlled “manual” type gearboxes were in the 1990ties assumed to be the obvious replacement for the automatic transmission due to its higher efficiency. Practical applications demonstrated that drivers did not accept the loss of acceleration torque that appeared during gear shifts. Different from conventional driving, where the gear shift is performed by the driver, the actuator driven gearboxes were supposed to change gear on their own initiative. This caused the gear shifts to occur unexpectedly, this being perceived as unpleasant.
If a gear shift can be performed during a very short interval, e.g. shorter than about 100 ms, the interruption of acceleration that occurs will not be noticed by passengers and the driver.
To perform a complete gear shift from the case when a continuous torque is exerted on the driving tires using a first gear position to a case when the same torque is exerted on the driving tires but using another gear, in a time period less than 100 ms is a challenging task. It requires that the driving torque is removed from the initial gear components, that the sleeve in the initial gear is removed, that the speed of the input shaft is changed to the speed required for the forthcoming gear, that the teeth of the sleeve for the forthcoming gear are aligned to the mating teeth of the forthcoming gear, that this sleeve is moved axially and that the driving torque is re-established. In an IEEE paper 07803-4943-1/98, “Advanced gear shifting and clutching strategy for Parallel Hybrid Vehicle with Automated Manual Transmission”, the obtained clutch released time during a gear shift is stated to be some 2 seconds.
To get an idea of the complexity of this undertaking, the delays in some mechanical control components can be illustrative. A small signal pneumatic relay has delays of some 10 ms. An electrically controlled hydraulic on-off valve has delays of some 15 to 30 ms. Obviously, the control of a complex series of events that is permitted to take some 60-90 ms cannot be arranged by a set of components, each with internal delays in the 10 to 30 ms range. The torque of electric motors can be changed in a fraction of a millisecond. Such motors that are always connected in the power train offer means that rapidly can affect the powertrain parts that are important for a gear shift.
One problem that makes fast gear shifts complicated is that the drive shafts that connect vehicle tires to the transmission are torsionally weak. The energy stored in their flexing can create considerable forces in the gear box of hybrid vehicles that have components with considerable inertia, such as electric motors, on the input shaft of the gear boxes.
Another problem is caused by the torque ripple damping device connected between the engine and the input shaft of the gear box. While essential to reduce noise and vibrations, during an acceleration it will store considerable energy that has to be released from the initial gear before a gear change. This energy also have to be restored after the engagement of the forthcoming gear to permit the engine torque to be transferred to the tires.
Conventional gearboxes as found in “manual” transmissions give lower losses than “automatic” transmissions. A hybrid vehicle topology “Strigear” disclosed in U.S. Pat. No. 6,740,002 for the same applicant has in simulations indicated a 37% fuel reduction when applied in a vehicle built with components and chassis closely resembling that of the first generation of Toyota Prius hybrid vehicles. This Strigear topology is based on actuator controlled “manual” gear box.
The very high importance of reducing the CO2 release from vehicles makes ultra efficient hybrids very important. The experience of customer reactions to automated “manual” gear boxes and the associated acceleration interruptions has created doubts on the possibility of a wide acceptance of the Strigear topology. To make the Strigear topology widely accepted, the gear shifts should preferably be made unnoticeable.